The Devil's Paintbrush by Jake Arnott

In March 1903 Major General Sir Hector Macdonald known throughout the Empire as "Fighting Mac" was in Paris fleeing an impending scandal. By chance he meets Aleister Crowley the notorious practitioner of the occult. Crowley then leads him on a night of drink and drugs during which Macdonald relives the key episodes of his life which have lead him to his present position. Arnott is, of course, well known for his crime novels but in this book he takes the reader on a masterly exploration of the psyche of a doomed hero.